A Group III nitride semiconductor (also called a Group III nitride compound semiconductor or a GaN semiconductor) such as gallium nitride (GaN) has been used widely as a material for various semiconductor devices such as a laser diode (LD) and a light-emitting diode (LED). For example, the laser diode (LD) that emits blue light is applied to a high-density optical disc or a display, and a light-emitting diode (LED) that emits blue light is applied to a display or illumination. Moreover, an ultraviolet LD is expected to be applied to biotechnology and the like, and an ultraviolet LED is expected as an ultraviolet source of a fluorescent lamp.
As a common method for producing a Group III nitride (e.g., GaN) crystal substrate, there is vapor phase epitaxy (hydride vapor phase epitaxy (HVPE)) such as halide vapor phase epitaxy (Patent Document 1) and metalorganic chemical vapor deposition (MOCVD), for example. On the other hand, as a method that can produce a Group III nitride single crystal of higher quality, a crystal growth method in a liquid phase is performed. This liquid phase epitaxy has had a problem in that the method was required to be performed under high temperature and high pressure. However, with recent improvements, the method can be performed under relatively low temperature and relatively low pressure and become a method suitable for mass production (e.g. Patent Documents 2 and 3). Furthermore, there is a method that uses liquid phase epitaxy and vapor phase epitaxy in combination (Patent Document 4).